1. Field of the Invention
The present invention relates to an optical disk, and more particularly to a shape of an information pit formed on an optical disk.
2. Description of the Related Art
In an optical disk, pits are carved on a transparent molded substrate at a part where data is recorded in advance, e.g., a pre-formatted portion of an RAM disk or an ROM disk. Such a pit is irradiated with a laser beam through a molded substrate from a surface opposite to the surface on which the pit is formed, and information is read.
Although the pit is formed with different sizes in accordance with recording information, its size is a sub-micron order. Forming a pit as small and accurate as possible is important to increase a recording density of the optical disk.
For example, in a current DVD-ROM, the shortest pit length (size in a circumferential direction) is 0.40 μm, a depth is approximately 100 nm, and a bottom forms a smooth conical trapezoid. In the conventional DVD, a time length of each pit is detected by slicing (binarizing) a reproduction waveform by a predetermined threshold value, and reproduces information by converting this length into data. However, in order to correctly reproducing information by this method, the pit length must be stably formed, and a signal amplitude which is sufficient for enabling slicing must be obtained. Therefore, the shortest pit shape must be conical trapezoid with a flat bottom, and this is one factor determining a limit of the recording density. A bottom circumferential size of the shortest pit is stipulated to, e.g., (0.2 to 0.25)×(wavelength)/NA/1.14 μm. Here, NA is a numerical aperture of an object lens.
As one conformation of an optical disk for coming generation, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-302310 proposes a mode of reading a signal through a cover layer having a thickness of approximately 0.1 mm.
The optical disk having such a conformation is the same as a conventional optical disk in that a reflecting film is formed on irregularities provided to the molded substrate and this film is irradiated with a laser beam in order to read a signal. In case of this conformation, however, as different from a conventional optical disk such as a CD or a DVD, the laser beam is transmitted through a cover layer instead of the molded substrate, and the reflecting film is irradiated with this laser beam.
The irregularities called the pit have a size of a sub-micron order, and how correctly the small pit can be formed is one important element which determines a signal quality and a recording density of the optical disk.
In the conventional optical disk which reads a signal by slicing a reproduction waveform by using a predetermined threshold value and binarizing it, the pit must be formed in such a manner that a reproduction waveform amplitude can be stably and largely obtained even in case of the shortest pit. Therefore, since a conical trapezoidal shape having a flat bottom surface must be assured even in the shortest pit, the recording density of the optical pit cannot be increased any further.
Further, like the prior art, when the shortest pit shape is formed into a conical trapezoid having a bottom surface and a large reproduction amplitude of a closest signal is assured, a difference between the shortest pit and the second shortest pit becomes small, and it is hard to discriminate a reproduction signal waveform of the shortest pit and a reproduction signal waveform of the second shortest pit. This becomes a serious factor of erroneous reading of a signal.
Furthermore, in a cover layer type disk that a reflecting film surface is irradiated with a laser beam instead of a substrate through a cover layer and information is read, or a Blu-Ray disc, when the shortest pit length is dense to the limit in order to increase the recording density, forming a metallic reflecting film may fill up the shortest pit in some cases. In such a case, there occurs a problem that a reproduction signal is deteriorated.